52 FORMS OF THE POLYPIDOMS. 



CHAPTER III. 



The Classifications of Zoophytes. 



The existence of a polypidom is not, as has been already 

 mentioned, essential to a polype ; nor does it exercise, when 

 present, that great influence over the organization of its archi- 

 tects and tenants which might have been anticipated. Thus 

 the animal of the madreporous Caryophylisea does not essential- 

 ly differ from the naked Actinia; and the gelatinous Hydra is 

 a true representative of the tenant of the sheathed Sertulariadse 

 and Tubularia. No ascidian polype, however, is ever found 

 detached, and without a polypidom ; and it is the same with all 

 our native Astroida, but, perhaps, the clustered animal-flower 

 (Actinia sociata, Ellis, Zoanthus, Cuv.) of the Caribbsean sea, 

 might take its place in this tribe with greater propriety than in 

 any other. 



In reference to their composition, Polypidoms may be divid- 

 ed into 1. the stony or calcareous, 2. the membrano-calcareous, 

 and 3. the horny and flexible ; but the line which separates these 

 divisions is often as uncertain and debateable as that which is 

 traced between the sister kingdoms. All are composed of the 

 same materials, viz. lime, and a gelatinous or membranaceous 

 substance ; and their peculiar characters depend on the differ- 

 ent proportions in which the materials are mixed. The calca- 

 reous, which are hard and inflexible, and, when dry, assume a 

 white colour, consist principally of carbonate of lime, with a small 

 quantity of the phosphate of the same earth, and the gelati- 

 nous matter which cements them into one coherent mass, is in 

 sparing proportion : that proportion is so greatly increased in 

 the polypidoms of the second section, that when the earthy in- 

 gredients have been removed by the action of diluted acids, the 

 structure retains its original form, and is, in fact, reduced to the 



